Firmware fuzzing is an effective technique for discovering vulnerabilities in embedded systems. However, existing coverage-guided firmware fuzzers typically obtain feedback through firmware instrumentation, hardware debug interfaces, or firmware rehosting, which requires access to the firmware source code or binary image. Such requirements are often infeasible for off-the-shelf embedded devices, where firmware binaries are inaccessible, unrehostable, immodifiable, or undebuggable, necessitating fuzzing under black-box conditions. In this paper, we present POZZER, a power side-channel-guided fuzzer for black-box embedded systems. POZZER uses power traces as feedback to identify previously unseen behavior via an incrementally constructed graph-based representation of observed executions, guiding the fuzzer toward unexplored execution paths. Its non-profiling design requires neither prior firmware knowledge nor a clone device, extracting meaningful feedback from a single power trace per execution while remaining robust to measurement noise. We evaluate POZZER on 15 firmware targets across two platforms and two real-world commercial embedded devices. Across the resulting target-platform combinations, POZZER outperforms a blind fuzzer under the same time budget in 26 out of 30 target-platform combinations. Furthermore, POZZER discovers two previously unknown vulnerabilities in one of the commercial devices, both confirmed by the vendor, demonstrating its potential for identifying vulnerabilities in black-box embedded systems.

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